I am really surprised at the quality of the answers offered. We undoubtedly got some Scholars here...!!
Just to add, as usual, and as it has already been put clear by the brothers, you cannot under any circumstance believe in 2 diametrically opposed beliefs. I cannot believe that someone could be a Muslim and a Christian at the same time. Let me mention one fundamental difference.
Islam says: Allah (God) is one and one alone, not part of a trinity.
Christianity says: (Even though some argue that God is one, but almost all of them believe...) God is part of the trinity, 3 Gods forming 1.
So you are a Chris-Muslim, which one would you stick to?!
It is just like you have turned Kafir.
And Allah say in the Qur'an:
"
Allah forgiveth not (The sin of) joining other gods with Him; but He forgiveth whom He pleaseth other sins than this: one who joins other gods with Allah, Hath strayed far, far away (from the right)".
And,
"
If anyone desires a religion other than Islam (submission to Allah), never will it be accepted of him; and in the Hereafter He will be in the ranks of those who have lost (All spiritual good)".
These are just one of the few Verses...
